256 



PLATHELMINTHES 



Like some flatworms they have a solid parenchyma bounded exter- 

 nally by a ciliated ectoderm rich in mucus cells, and inside this at least two 

 muscular layers, an outer circular and an inner longitudinal layer. They 

 differ from all other Plath.elminth.es in having a complete alimentary 

 tract, beginning with a ventral anterior mouth and continuing as a 

 straight tube, with, usually, paired diverticula, to the vent at the posterior 

 end of the body (fig. 235). 



Especially diagnostic is the proboscis, which lies dorsal to the alimen- 

 tary tract and usually opens in front of the mouth. The proboscis is a 



p ps pm 



Iv di 



FIG. 235. Diagram of Nemertean forig.). b, brain; c, ciliated pit; d, dorsal nerve 

 trunk; di, dorsal blood-vessel; g, gastric caeca; i, intestine; /, lateral nerve trunk; h>, 

 lateral blood-vessel; p, proboscis retracted; pm, proboscis muscles; pr, protonephridial 

 tube; po, its opening; ps, cavity of proboscis sheath. 



muscular tube closed at one end and at rest is infolded like the finger of 

 a glove inside a closed sac, the proboscis sheath, which extends far back 

 in the body. Its tip is bound to the posterior end of the sheath by a 

 retractor muscle. By contraction of the sheath the proboscis is everted, 

 while it may be retracted again by the muscle. Nettle cells are not un- 

 common in the proboscis wall, while in some- forms (the older Enopla) 

 the effectiveness of the organ is increased by the presence of a dart-like 

 stylet at the tip (reserve stylets occur on either side, fig. 236), and at the 

 base of the stylet is a poison sac. 



The blood-vascular system consists of a pair of lateral tubes connected 

 by transverse loops, and in most forms a third tube is present lying between 

 the intestine and the proboscis sheath. The blood is colorless; it rarely 

 contains red or green corpuscles. 



The central nervous system (in some forms still in the ectoderm) con- 

 sists of a supracesophageal brain of a pair of ganglia, from which nerves 

 run to the proboscis, and two lateral cords united on the ventral side by 

 numerous transverse commissures. Connected with the brain, either 

 directly or by means of a short nerve, are the cerebral organs or ciliated 

 grooves, pits on the sides of the head, formerly regarded as respiratory, 

 are now considered sense organs. Tactile organs and simple eyes are 



